You can first find this quest once the waters in the Lake of Nine recede for the second time. As you head up the channel to the North you'll enter The Mason's Channel, where a Restless Spirit will ask that you find the bones of his beloved Gullveig that have been scattered across the Lake of Nine.

The first piece of remains can be found on the shores below The Council of Valkyries. Take the boat to dock below the Alfheim Tower, and look for a ledge blocked by rocks you can blow up with a nearby red urn.

Hop up on the ledge and look along the left wall along lots of seaweed, and you can find some bones sticking out to loot.

The final piece of remains lie in the Northwest, on a small island guarded by an Ancient - A powerful icy Soul Reaver. In fact, the lumbering golem is pretty docile, as long as you don't get close, and you can scout by the shore if you don't wish to fight and find the last piece.

Return to The Mason's Channel and defeat any Tatzelwurms that appear before speaking to the spirit. You'll find that Gullveig is in fact, a very powerful Revenant that will waste no time in attacking you, and summoning some Level 5 Nightmares to help do the job.

Don't be fooled by the frail appearance of Valkyries, Gullveig can deal a lot of damage with quick jabs from her staff that are hard to spot - and you'll need to constantly have Atreus stun or shock the Revenant to give yourself an opening, and follow up it's lowered defenses with a big runic attack.

Kill Gullveig, and you'll receive an Offering to Tyr - a component needed to help craft the Armor of Tyr.